Case Study: Design of Lightning Protection Circuit for LED Lighting

As we all know, the 2ProAC device integrates the PolySwitch polymeric positive temperature coefficient (PPTC) overcurrent device and metal oxide varistor (MOV) element in an innovative thermal protection device for overcurrent protection and voltage clamping in overvoltage faults. Using this single device can help manufacturers meet industry testing requirements, reduce component count and improve equipment reliability.

　　A common application of 2ProAC devices is LED lighting , where the lightning protection requirement is less than 2kVsurge, and 2ProAC can protect against 2kV lightning strikes. The figure below is a schematic diagram of a customer's LED circuit board debugged by a netizen not long ago. The lightning strike requirement is 2kV. The 2ProAC is placed at the input end of the LED lighting circuit, and a lightning waveform generator and AC coupling are added.

　　But something strange happened! After the 2kVsurge, the LED light went out. Since the circuit board was still connected to the 220V power supply, it was obvious that the 2ProAC did not protect the driver chip of the subsequent stage. After checking the driver chip, it was found that it was indeed damaged. Later, it was reduced to 1kVsurge, but it still could not pass. The driver chip was still damaged. The author analyzed the customer's circuit and found that the topology of the customer's circuit adopted a single-stage PFC structure. The advantage is that the circuit is relatively simple and can achieve a higher PFC value. However, the circuit does not use the common electrolytic capacitors in the power switch circuit (generally about 47uF/400V), but instead uses high-voltage capacitors with small capacitance values. It is precisely because of the absence of this large electrolytic capacitor that the lightning protection ability of the single-stage PFC structure circuit is greatly reduced, and the driver chip is damaged when the lightning surge occurs. It can also be said that 2ProAC is coordinated with the large electrolytic capacitor to pass the surge pulse, and it also protects the electronic circuit of the subsequent stage. The lack of any protection performance will be greatly reduced.

　　From the above case analysis, it can be seen that the 2ProAC device passing the customer's surge requirements is closely related to the client's circuit topology. The client's PCB board needs 2ProAC for lightning protection, and 2ProAC also needs the client's circuit (such as electrolytic capacitors) to work together to protect the fragile electronic circuits of the subsequent stage. Therefore, a reliable circuit lightning protection solution must be implemented according to the customer's specific circuit and application.